The present invention relates generally to gas turbine engines, and, more specifically, to turbine blades therein.
In a gas turbine engine, air is pressurized in a compressor and mixed with fuel in a combustor for generating hot combustion gases from which energy is extracted in turbine stages disposed downstream therefrom. A high pressure turbine (HPT) powers the compressor through one drive shaft, and a low pressure turbine (LPT) powers an upstream fan in a turbofan engine application, or powers an external drive shaft for marine and industrial applications.
Each turbine stage includes a turbine nozzle in which a row of stator vanes direct the hot combustion gases downstream through a row of turbine rotor blades mounted to the perimeter of a supporting rotor disk to power the drive shaft. The turbine blades include airfoils extending in radial span from root to tip and axially between opposite leading and trailing edges.
Each airfoil has a generally concave pressure sidewall and a circumferentially opposite, generally convex suction sidewall spaced transversely apart to define an internal cooling circuit. The cooling circuit typically includes several radial channels or passages separated by longitudinal partitions or ribs.
The cooling circuit is fed with pressurized air bled from the compressor which is channeled through inlets in the supporting dovetail of each blade to carry the cooling air radially outwardly through the airfoil during operation.
The individual cooling passages in the airfoil terminate at the radially outer tip cap of the airfoil, which typically includes outlet holes therein for discharging a portion of the internal cooling air. The airfoil typically includes various rows of film cooling holes through the pressure and suction sidewalls, as well as a row of trailing edge outlets or slots which collectively discharge the spent cooling air from the airfoil and provide thermal protection thereof.
The internal cooling circuit may have various configurations for differently cooling the different portions of the airfoil between the leading and trailing edges and along the opposite pressure and suction sidewalls. Dedicated cooling passages may be located along the leading edge and along the trailing edge, with different cooling passages located axially therebetween.
For example, the turbine airfoil commonly includes one or more serpentine cooling circuits having an outbound inlet passage extending to the airfoil tip which then changes direction in a flow bend into a radially inbound flow passage extending to the airfoil root, which yet again changes direction in another flow bend into another radially outbound flow passage in a three-pass serpentine circuit.
The modern turbine blade is typically manufactured by casting which requires a ceramic core to define the intricate features of the internal cooling circuit inside the blade. The casting process and configuration of the several radial passages inside the airfoil typically result in substantially flat or horizontal inner surfaces of the tip cap above each of the flow passages.
As indicated above, the tip cap may include small outlet holes for discharging a portion of the spent internal cooling air out the airfoil tip during operation in conjunction with discharge of the air through many rows of film cooling holes.
However, the cooling air may include small particles of dust in various quantities depending upon the specific environment in which the engine is operated. For example, a turbofan aircraft engine may be used power an aircraft in flight through various locations in the world, some of which are prone to significant atmospheric dust especially around landing fields.
Accumulation of dust inside the small passages and holes of a turbine blade is a well known problem which is typically ameliorated by introducing relatively large dust holes in the tip cap of the turbine blades. The size of the dust holes is typically about twice the size of the common film cooling holes found in the airfoil sidewalls, and correspondingly increases the flow discharge therefrom, but with the significant attribute of carrying therewith and discharging from the turbine airfoil significant quantities of the entrained dust.
Accordingly, a compromise is made in the tip cap region to locally increase the flow discharge at relatively few dust holes for the benefit of reducing dust accumulation inside the airfoil.
However, experience has shown in one turbine blade used publicly in commercial use throughout the world for many years that despite the use of such dust holes in the blade tip cap, dust may still accumulate under the tip cap and eventually block discharge flow through the dust holes leading to a shorter life for the turbine blades.
Experience and actual observations of turbine blades used in service show dust accumulation at the radially outer end of the internal flow passages, including both serpentine and non-serpentine flow passages. It appears that the initially fine dust particles entrained in the cooling air accumulate or aggregate together during operation to form larger particles or chunks.
Centrifugal force drives these particles and chunks radially outwardly during operation, which are then trapped by the inner surface of the tip cap. The particles and chunks may then bond to the inner surface of the tip cap.
Or, some of the chunks may remain loose and upon shutdown of the engine these loose chunks may then drop toward the roots of some of the blades until the engine is next again started, in which case the chunks are again driven radially outwardly and may fully block the relatively large dust holes themselves. Upon blocking of an individual dust hole, the dust particles entrained in the cooling air may then accumulate below the tip cap and further increase the flow blockage and eventually completely block cooling flow in an individual flow passage.
Accordingly, it is desired to provide a turbine blade having an improved dust extraction configuration for increasing the useful life of the blade in dust environments.